Fall Officially Begins

Fall Officially Begins at 8:09pm local time on Wednesday, September 22 -- a good excuse to throw an impromptu evening party to celebrate the occasion. At equinox, the sun shines directly down somewhere on Earth's equator. At 8:09pm San Diego time this year, that equatorial spot lies in Indonesia. The autumn season will continue for another three months until the sun "moves" to its farthest southerly point, winter solstice, December 21.

Equal Days and Nights everywhere on Earth, 12 hours each, are only one noticeable consequence during the time of equinox, either autumnal or vernal (spring). Another consequence is that the sun at equinox always rises from a point on the horizon due east and later sets due west. You could calibrate a compass this way if you had access to a true (unobstructed) horizon. Another, subtle consequence is that at mid-latitudes like ours, morning and evening twilight periods are shortest during equinox. From San Diego, the duration of twilight this week is about 80 minutes; last June it was about 100 minutes.